1. Field of the Invention
The present invention relates to a process for the preparation of a shaped substrate for a coated conductor, in particular to a process allowing improved freedom of shaping of the substrate, e.g. into a round wire, and consequently of the coated conductor as well as to a coated conductor using such substrate.
2. Description of Related Art
Coated conductors have a shape of long length such as a tape or stripe. Generally, they are composed of a substrate, an active layer of high-temperature superconductor material and a varying number of buffer layers between the substrate and the superconductor layer. The buffer layers serve to compensate for the various different properties of the materials used.
Though not restricted thereto currently the rare earth barium cuprate-type superconductors of the formula REBa2Cu3O7-x are conventionally used in the production of coated conductors. A particular member thereof is that one known by the reference YBCO-123 wherein the numerical combination 123 stands for the stoichiometric ratio of the elements Y, Ba and Cu.
A major problem in the production of coated conductors is the crystallographic orientation of the crystal grains of the superconductor material.
In order to have good superconducting performance, for example in terms of critical current density (Jc) and critical current (Ic), the superconductor material should have a high degree of orientation or texture with the individual crystal grains being oriented essentially in parallel to each other and with an inclination against each other as small as possible.
Preferably, the superconductor layer has a biaxial texture with the crystal grains being aligned both in the same direction with respect to the surface plane (a-b alignment) and perpendicular to the plane (c-axis alignment).
The quality of the biaxial texture is typically expressed in terms of the crystallographic in-plane and out-of-plane grain-to-grain misorientation angle which reflects the degree of inclination of individual crystal grains against each other. The smaller the misorientation angle the better (“sharper”) the texture of the layer.
Usually, the degree of texture is determined by X-ray diffraction specifying the in-plane and out-of-plane orientation distribution function of the grains of the layer.
Based on the X-ray data the values of the full-width-half-maximum (FWHM) of the in-plane phi scan (Δφ) and out-of-plane rocking curve (Δω) can be obtained. The smaller the respective FWHM-value the sharper the texture.
Orientation of a layer to be grown can be achieved by epitaxial growth. Epitaxial growth refers to a process wherein the layer to be grown adopts the crystallographic orientation of the substrate or layer onto which it is grown.
There are two types of epitaxy: homoepitaxy refers to the growth of a layer onto a substrate of the same material, and heteroepitaxy refers to the growth of a layer onto a substrate of different material.
That is, the crystallographic orientation of the layer grown is directly related to the crystallographic orientation of the underlying layer onto which the layer is deposited.
Consequently, for the quality of orientation (texture) of a layer to be grown epitaxially the quality of orientation of the underlying layer, the template layer, is decisive.
There are currently two main approaches to achieve the required texture. According to the first approach a highly textured buffer layer is deposited onto a polycrystalline, randomly oriented substrate by directed physical coating processes requiring high vacuum such as ion beam assisted deposition (IBAD). The highly textured buffer layer serves to transfer the desired texture to the superconductor layer grown onto the buffer layer. Such high vacuum deposition techniques requires expensive equipment. Further, coating of substrate of long length is difficult.
According to the second approach a highly textured substrate is used which can be obtained by mechanical working, for example by RABiTs (rolling assisted bi axial texturing of substrates). Here, the texture of the substrate is transferred to the buffer layer and, then, to the superconductor layer deposited thereon. Since this approach uses epitaxial growth it is no longer necessary to apply a directed deposition process such as IBAD for obtaining a buffer layer of desired orientation.
The present invention relates to the second approach based on a suitably textured substrate.
There is a plurality of deposition methods known for growing buffer layers onto a (biaxially) textured substrate.
Examples are vacuum processes such as pulsed laser deposition, physical vapor deposition, electron beam evaporation and sputtering, as well as non vacuum processes such as metal organic chemical vapor deposition (MOCVD) and chemical solution deposition (CSD).
According to the present approach of coated conductor production the quality of the texture of the substrate is an essential feature for allowing the formation of an active layer of high-temperature superconducting material of good superconducting performance. If the texture of the substrate is poor it is not possible to obtain an active layer with the desired well aligned orientation which is a prerequisite for superconducting properties.
However, there is the drawback when using a textured substrate that the final shape of the coated conductor is already fixed.
Once a substrate has been textured it is no longer possible to shape it by deformation and annealing, because defects introduced during shaping deformation annihilate during subsequent annealing thereby destroying the texture. However, as set out above, it is not possible to grow a well oriented layer, buffer layer and active layer, respectively, by epitaxial growth onto a substrate or underlying layer with poor and/or defective texture.
Otherwise, currently texturing of the substrate is carried out by mechanical processing which requires a planar and regular surface. Consequently, if the substrate is shaped prior to processing, for example to a curved irregular shape such as a round or polygonal shape, mechanical processing for texturing is difficult or, even, no longer possible.
This means that in current processes forming of a once textured substrate is avoided which restricts applications to those adapted to the planar tape-shape of the substrate and which considerably restricts the field of application.
U.S. Pat. No. 6,114,287 relates to a method for mechanically deforming a ductile epitaxially deposited metallic buffer on a textured surface to minimize or eliminate surface irregularities while maintaining the biaxial texture of the buffer layer. The method includes the steps of depositing an epitaxial layer of a metallic buffer on biaxially textured substrate; and deforming the epitaxial layer between smooth surfaces. Thus, a general object of the patent is to smooth an epitaxially deposited layer on a textured substrate.
Another object of the US patent is to provide a method for preparing a substrate having a densified HTS precursor by densifying between smooth surfaces. For elimination of the surface roughness or densification the coated substrate is mechanically deformed between smooth surfaces such as rolling between polished rolls or pressing between polished platens.
In the method of U.S. Pat. No. 6,114,287 the treated coated substrates maintain their initial planar geometry.
WO 03/019589 A1 relates to a process for obtaining a coated conductor in coil configuration.
It is considered difficult to obtain such a coil configuration by simple winding of a coated conductor tape due to the brittleness of the layers.
According to WO 03/019589 A1 this problem is overcome by direct deposition of the respective layers in coil configuration onto a former. That is, the coil configuration is “written” onto the former by a deposition technique such as IBAD. There is no indication as to shaping a coated substrate, i.e. deforming the coated textured substrate from a first geometry to a second geometry. Moreover, there is no indication to subject the coated substrate to a shaping step that is, shaping the substrate and the buffer layer simultaneously.
DE 197 24 618 A1 relates to a process for preparing a corrugated metal tube having a layer of oxide ceramic superconductive material by bending a planar metal substrate coated with the superconductive material along its longitudinal axis to obtain a slit tube and welding the slit. There is no indication as to any texture of the substrate and/or the superconductive layer. Further, such oxide ceramic superconductive material is very liable to damage on deformation.
Object and Summary
It was the object of the present invention to provide a process for producing a coated conductor using a textured substrate which process allows forming of the already textured substrate and, nevertheless, allowing epitaxial growth of a well aligned active layer of high-temperature superconducting material.
In particular, it was the object of the present invention to provide a process for preparing shaped substrates for epitaxial growth of a superconducting phase.